The present invention relates to demetalization of portions of metalized plastic foils for purposes of using the foils in electrical capacitors.
Metalized plastic foils, ribbons, strips or the like are used as components in electrical capacitors wherein any coherent metallization establishes an electrode and the plastic is the dielectric material. For a variety of reasons a portion of such plastic foil, e.g. an edge strip has to be free from metalization. One way of obtaining such free areas is to prevent metalization thereof during the metalization step, e.g. by means of covers, strips, masks, etc.
Another method, broadly, involves the local demetalization of foil, which was metalized initially in its entirety. For example, one can burn off electrically a strip portion of the metalization. German Patent No. 425,870 and printed application Nos. 2,509,543 and 897,881 disclose mechanical removal of the metalization through scraping, brushing or the like. German printed patent application No. 1,938,320 describes the use of rubber covered disks cooperating with a counter roll over which the strips to be demetalized passes, and the rubber, so-to-speak, erases the metal from the foil. By and large the controlled subsequent local demetalization of completely metalized foil was found to be a more economical procedure than the selective local prevention of metal deposits during the metallization. However, the known methods of local demetallization have a number of insufficient draw-backs.
Burning off of metal by means of electrodes under electric tension is not suitable for thermo-plastic foil because the plastic deforms. Scrubbing, scraping, brushing or the like is better on that account as long as the tool friction does not heat the foil unduly. Also, using bonded corundum granules for scraping or finishing narrow strips is difficult because the corundum particles break or break off. Thus, these known mechanical techniques of metal removal have not yet been successfully practiced. The "erasing" technique is not practical because the rubber wears off very rapidly.